1. Field of the Invention
This invention is directed to a process for preparing phthalide by reaction of 3-chlorophthalide with hydrogen. More specially, this invention is directed to a process for the synthesis of high quantities of phthalide by a catalytic reaction of 3-chlorophthalide with hydrogen wherein hydrogen chloride which forms as a by-product is effectively removed. The reaction takes place in accordance with the following equation: ##STR1##
2. Discussion of the Prior
The preparation of phthalide has been performed hitherto either through the reaction of 0-disubstituted benzenes in a plurality of difficult steps with, to some extent, unsatisfactory yields, or through the reduction of phthalic acid anhydride in the presence of catalysts. In these catalytic reactions, the water that forms interferes with the reaction and leads to the formation of undesirable by-products which contaminate the phthalide. These disadvantages are partially avoided in the electrochemical reduction of phthalic acid anhydride or ammonium phthalamale (German OS No. 21 44 419), but the performance of these electrochemical processes generally presents technical difficulties and involves a relatively large investment in apparatus.
The replacement of organically bound halogen with hydrogen, hereinafter referred to as hydrogenolysis, in the presence of catalysts of Group VIII of the Periodic System is also known in itself. In this process, aromatically bound halogen is more easily substituted than aliphatically bound halogen. In all cases the reaction takes place all the more easily the less acid the reaction solution is. However, since hydrogen chloride is released in the hydrogenolysis, known hydrogen chloride acceptors are generally added to the reaction mixture to improve the reaction rate and the space velocity.
The addition of acid-binding compounds of this kind, such as, for example, amines, sodium acetate, and alkail-containing methanol, has nevertheless the disadvantage of complicating the working up of the reaction product and of the catalyst. In particular, the recovery of the catalyst essential to the process can be accomplished only by a plurality of procedures, so that such a method of preparation is not technically feasible.
Another method of intercepting the hydrochloric acid consists in performing the hydrogenolysis in the presence of a solvent having a high dissolving power for hydrogen chloride. Methanol, for example, is suitable as such a solvent in the hydrogenolysis of benzyl chloride to toluene.
If it is desired to apply this method of hydrogenolysis in the presence of methanol to the preparation of phthalide from 3-chlorophthalide, however, several disadvantages are encountered. The phthalide reacts further in the presence of hydrogen chloride and methanol with the cleavage of the lactone ring to form o-hydroxymethylbenzoic acid ester, o-chloromethylbenzoic acid ester and o-toluic acid methyl ester, so that phthalide is produced in a yield of only 15 to 18%.
It has also been proposed to react molten or dissolved chlorophthalide catalytically with hydrogen at elevated temperatures in the absence of a hydrogen chloride acceptor, allowing the gaseous hydrogen chloride to escape from the reactor. The disadvantages of this proposal are evident. If a catalyst supported on a support material of the conventional kind is used, the catalyst has to be kept in a quasi-homogeneous suspension in the liquid phase under the conditions of the reaction and must be vigorously mixed with the hydrogen phase in the reaction vessel by stirring. In order to be able to achieve this quasi-homogeneous suspension of the catalyst grains, a grain size spectrum of from 0.05 to 0.3 mm is indicated for the catalyst. Now, the required vigorous mixing of the liquid and gas phase in the reactor brings about a comminution of the catalyst grains. Furthermore, catalyst losses occur during the filatration of the catalyst from the reaction product. Operation with granular catalyst in the sump phase is therefore uneconomical due to the catalyst losses which occur during the necessary filtration. If one operates without solvent, the difficulties that are involved in the filtration increase.